Tires are reinforced with belts or cords made of metal, above all in the area of the tread. In order to check whether these belts or cords are correctly inlaid, the tires are X-rayed for checking with X-rays in order to visualize the belts or cords located in the tires and the images thereby obtained are evaluated in order to identify deviations from a desired standard.
In the X-ray inspection of radial and diagonal tires, the X-ray line images are produced by means of a wide-angle X-ray tube and a U-shaped X-ray detector array. Typically, in a line the tire is to be viewed from bead core to bead core. While the tire rotates, individual lines are recorded. These are referred to as X-ray line images in the context of the present application. These are then put together to form an overall image that depicts the whole tire.
There are two basic problems with this type of image acquisition. Through the projection of a three-dimensional object onto a two-dimensional X-ray line image, on the one hand items of location or depth information are lost in the beam direction and on the other hand, because of the complex imaging geometry, items of position and distance information cannot be directly determined as physical variables in the X-ray line image.
An essential inspection criterion in the X-ray inspection of tires is assessment of the location of the different tire components with respect to each other, in particular of the steel belts or the cords of which the steel belts consist. For example, the distance between two belt edges must not lie outside a predefined tolerance range. Tolerance ranges of this type are generally specified as physical variables and usually contain an accurate definition of the measurement points to be selected. Because of the problems mentioned, neither a free selection of the measurement points nor a direct assessment of the location is possible.
In a valuable contribution to the state of the art, DE 10 2013 001 456 B3 describes a method for calibrating an X-ray inspection system for a tire type. The items of position information on tire components are first determined automatically or manually in the form of image coordinates in the X-ray line image and then converted to physical variables by means of a tire-type-specific geometric calibration. A geometric calibration assigns to each detector pixel a physical height and width in the X-rayed object. The distance between two measurement points defined in the X-ray line image results from the combination of the pixel heights and widths along a path between the measurement points. The measurement points cannot be freely selected but implicitly lie in the calibration plane. For example, a determination of the horizontal distance between two tire components in relation to a radial section is thereby not possible or is affected by a systematic deviation.